The atomization stage in spray drying produces a spray of droplets from liquid/slurry bulk having a high surface to mass ratio. The dried product that results from evaporation of the liquid part in the atomized spray can be made to possess the desired particle size distribution through control of the atomization variables. There are usually two types of atomizers, one rotary type and the other one is nozzle type. With rotary atomizers, centrifugal energy is utilized. This adopts a rapidly moving centrifugal disc/wheel with peripheral speed exceeding 200 m/s. Here the centrifugal force is used to break the slurry/liquid introduced through vaned wheel on disk which is ejected out in the form of thin sheet of liquid. Feed, on leaving the periphery, readily disintegrates into a spray of droplets. These types of atomizers require high rotating speed of the disc/wheel (up to 20,000 rpm in industrial scale) (Atomizers are found in the range from 3000 to 50,000 rpm). Also they require a larger diameter of drying chamber in view of projection of the sprayed particles taking place in horizontal direction. The maintenance cost of the high speed rotating parts is also high.
In case of nozzle atomization, high hydraulic pressure is used to spray slurry/liquid through an orifice. Here the pressure energy within the liquid bulk is converted into kinetic energy of thin moving liquid sheets. The sheets break up (i) under the influence of the physical liquid properties and (ii) by the frictional effects with the medium into which the liquid sheet is discharged. Invariably the medium is air. The drawback of this type of atomizers is that they need very high pressure in the range of 27-690 bar. Also in this case due to high pressure the nozzle wears very fast, especially when abrasive materials are used.
The conventional art of rotary and nozzle type atomizer used in spray dryers have been exhaustively dealt by the following:                1. Perrry's Chemical Engg. Hand book, Internat. Edn., Mc Graw Hill, p. 12.81-12.90, 1997.        2. K. Masters, Spray drying Hand book, The Pitman Press, Bath, UK, p. 3-6, 33-45, 1979.        
Fred V. Shaw (Ceram. Bull, vol. 69, 1990, p. 1484-1489 describes the atomization process, basic spray drying process, mixing and drying in a spray dryer, powder separation, ancillary equipment and proper design requirement.
David Lee (Ceram. Bull., vol. 53, 1974, p. 232-234) has made a clear comparison between centrifugal and nozzle atomization in spray dryers with regard to resultant product characteristics, maintenance, design consideration and actual operations.
David Oakley (Chem. Engg. Progr., October 1997, p. 48-54) discussed the system design of spray dryers and concluded that rotary atomizers are chosen for small to medium size particle production where as pressure nozzle atomization are adopted for medium to large particle size powder preparation.
Zhao, Dowson and Jacobs, in 2000 (Modelling Simul. Mater. Sci. Eng. vol. 8, p. 55-65) have made modelling of liquid flow after a hydraulic jump on a rotating disk prior to centrifugal atomization.
U.S. Pat. No. 4,562,966 of Jan. 7, 1986 (Donald A. Smith & Richard C. La Flesh) describes the design of an atomizer based upon existing dual fluid atomizer technology (not fully inherent) for production of a dispersed spray of substantially liquid material (coal-water slurry) for augmenting the dispersion of liquid material as a spray. The nozzle diameter of the pressure nozzle type atomizer is chosen 10 times the diameter of largest particle present in the slurry and outlet pressure of slurry is maintained in the range 690-1380 kPa (6.9-13.8 bar). 70-110 μm diameter droplets are produced when air to fuel mass ratio is 0.1. The atomizer shows similar performance when 50% fuel mass flows at 0.3 and higher air to fuel ratio.
U.S. Pat. No. 4,383,649 of May 17, 1983 (Robert D. Reed, Richard R. Martin, Hershel E. Goodnight) describes an improved fuel oil pressure nozzle atomizer. Steam and oil flow under pressure coaxially to mix thoroughly at the central bore of the burner. Two factors have been identified to be important: (a) Central bore of burner must be of constant diameter from the point where the steam and oil mix, outwardly towards the burner tip ports (b) The total cross-sectional area of the tip ports must be less than the cross-sectional area of the central bore. The main object of this invention is to provide a improved oil fuel atomizer, which, on a pound for pound ratio of steam to oil provides a satisfactory flame, with minimum ratio of steam to oil.
U.S. Pat. No. 4,169,556 of Oct. 2, 1979 (Horst Muller) reports the design of a flat discharge device (pressure nozzle atomizer type) comprising separate inlet for liquid and solid particles and a tubular mouth piece of flat cross-sectional shape in which the solid particle and the liquid are to be mixed and ejected from the front face of the mouth piece. Wear of narrow side wall is of the mouth piece is claimed to be minimized by locating the solid particles completely within the liquid jet without touching the side walls. Liquid used is water and solid used are aluminium slag, quartz sand powder, chalk and rust particles.
An old Patent GB787934 of 1957 (American Cyanamid Co.) dealt with a centrifugal type of atomization in which an aqueous dispersion of hydrated silica containing catalyst material is sprayed outwardly from an axis of rotation while simultaneously impinging there on a set of convex surfaces moving in a circular path around said axis and discharging the material from the said surfaces in to a current of hot gases and drying resulting particles by contact therewith. Various prior art references, e.g. U.S. Pat. Nos. 3,032,275, 2,606,073, 4,252,276, 4,087,050, 3,920,187 describe atomizer for spray drying.
The above cited works have either used high pressure or high speed (rpm) to atomize slurry or solutions. In case of nozzle atomizers they require high pressure compressor and ducting system. Also, here nozzle wears very fast. On the other hand, the rotary atomizers require a larger diameter of drying chamber. No one has so far tried to simultaneously apply pressure nozzle with centrifugal wheel to atomize in a spray dryer. In order to overcome the cited shortcomings of the conventional rotary and nozzle type of atomizers, a novel atomizer has been designed.
The present invention of jet-wheel impact atomizer uses the simultaneous application of pressure nozzle and centrifugal wheel in one device for spray drying while employing low pressure as well as low wheel speed.
The free flowing alumina powder finds use in ceramics, electronics, thermal and electrical engineering.
Powders, in general, are characterized by irregular shaped grains. When subjected to stress, these grains mechanically interlock among themselves and prevent the free flow of powder. Free flowing powders of ceramics like alumina, zirconia, titania etc. are required for such applications as compaction, extrusion, plasma and high velocity oxygen (HVO) spray, thermal and electrical barrier coating, catalyst and catalyst support, high temperature components and parts etc. Alumina powder produced in large industrial plants (by Bayer's process) require granulation or spheroidization for filling mould or die uniformly with a view to achieve high densification (>90%) without voids and defects for manufacture of electronic component such as IC substrate and high density sintered products. Once the grains are made rounded or spheroidised, they become easily flowable or free flowing. At stage I of compaction, maximum densification is achieved by free flow property of powder.
Spheroidization or micro granulation of powder grains is widely done by spray drying of a slurry or sol in a drying chamber. Due to the effect of high velocity in spray and surface tension of the liquid, the slurry jet disintegrates into a large number of minute spherical droplets or atomized particulates which when undergo simultaneous drying at high temperature (300-500° C.), produce spheroidised or rounded particles or grains. Conventionally, there are two types of spray dryers: (i) Pressure nozzle type, (ii) Centrifugal or rotary type. Where as the pressure nozzle type spray dryer uses high slurry pressure (27-690 bar), the centrifugal type spray dryer employs high wheel speed up to 20,000 rpm in industrial practice. The jet-wheel impact atomizer based spray dryer, however, gets rid of the requirement of the above high slurry pressure and high wheel speed and operates within 5 bar slurry pressure and its wheel rpm ranges between 6000-14000, thus causing a considerable saving of energy in spray drying process and producing powder of different grain sizes. The present invention describes the preparation of free flowing alumina powder from suspended alumina-water slurry by adopting the jet-wheel impact atomization based spray drying process. Grain spheroidization or rounding has resulted in the free flow property and different particle sizes in alumina powder.
Masters' treatise on spray drying (Spray drying Hand book, Pitman, UK, 1979, page 33-45) discusses various aspects of spray drying with theoretical background. The Perry Chemical Engg. Hand book (McGraw Hill, 1997, page 12.81-12.90) illustrates and discusses the industrial designs of co-current and counter current spray dryers using high pressure nozzle and rotary type atomizers. James S. Reed has extensively dealt ceramic powder processing including spray drying, in his book ‘Principle of Ceramics Processing’, Second Edition, John Wiley, 1995, page 378-393.
K. Y. Shue et al. (Int. J. Powd. Met., vol. 31, 1995, page 145-153) have discussed centrifugal atomization of aluminium and the resulting morphology of the particles.
D. C. C. Lam et al. (J. Mater. Sci., vol. 30, 1995, page 5495-5501) have taken 28 and 32 vol % of alumina with different combination of binder and plasticizer and spray dried at 1.38 kPa pressure and 75° C. to produce alumina granules and studied their mechanical and microstructural properties in green bodies.
David E. Oakley (Chemical Engg. Progr. October, 1997, page 48-54) has dealt the conditions under which uniform particles can be prepared by rotary and pressure nozzle atomizers. One co-current rotary atomizer and three counter-current pressure nozzle atomizer have been discussed.
Kim and others (Chem. Mater. Vol. 14, 2002, page 2889-2899) have reported the synthesis of loosely agglomerated spherical nano porous alumina particles by spray drying a solution of aluminium nitrate and sodium chloride at 220-500° C. A. Kumar et al. have recently (J. Amer. Ceram. Soc. vol. 84, 2005, page 971-973) described the laboratory scale spray drying of precursor solution at 140 kPa pressure and a temperature of 500° C. to prepare spheroidised La0.84Sr0.16MnO3 powder.
U.S. Pat. No. 5,302,368 of Apr. 12, 1994 (Harato et al.) describes a process where an aqueous slurry of about 100-1000 cP viscosity suspending about 200-2000 g/lit. of aluminium hydroxide particles smaller than 5 μm size is spray dried, and the dried powders are calcined to obtain alumina powders. The alumina powders have a sharp particle size distribution, being suitable for electronic, abrasive and refractory applications. US Patent Application No. 20020193236 of December, 2002 (M. Takaya et al.) describes a process for manufacturing spherical ceramic powder by spray drying. The process essentially consists of spraying a ceramic slurry through a nozzle at a temperature >100° C. and the liquid content of the sprayed droplets are removed to obtain granular powder. The mean particle size is found in the range 1-50 μm with a sphericity of about 0.8 or higher which is suited for mixing with resin material to form a compound. Various prior art references, e.g. U.S. Pat. No. 3,966,644 of 1976, U.S. Pat. No. 4,649,037 of 1987, U.S. Pat. No. 4,713,233 of 1987, U.S. Pat. No. 5,972,835 of 1999 describe the spray drying of alumina and other ceramic compounds to produce spherical powders.
The disadvantages of the above referred works are high erosion rates of the nozzles when abrasive slurry like alumina is used and also the relatively higher rpm of atomizer wheel (in case of centrifugal atomization) leading to higher energy consumption.
No work has so far been reported on the preparation of free flowing alumina powder produced by jet-wheel impact atomization based spray drying, a novel process that overcomes the above shortcomings.
The novelty of the invented process lies in the idea that through impact of a slurry/liquid drop against the spinning wheel moving opposite to the direction of slurry/liquid jet motion, transfer of momentum to slurry/liquid drop can take place up to 2 times (maxm.) that of the original momentum value. Kinetic energy for disintegration of drop will be available by 4 times (kinetic energy=p2/2 m) more than the normal collision case (this takes place ideally for an elastic collision, some energy loss due to friction always takes place at nozzle and its orifice). Thus, at low slurry/liquid pressure, the slurry/liquid drop can be broken or atomized to thousands of tiny droplets without going to high pressure (29-690 bar in pressure atomization spray dryer) and high wheel speed (up to 20,000 rpm in centrifugal atomization spray dryer adopted in industrial production). The novelty of the process lies in the fact that unlike conventional spray drying (high pressure nozzle and rotary type atomizer based), it does not use high pressure slurry (29-690 bar) and high atomizer wheel speed (which goes up 40,000 rpm) but employs a unique combination of low slurry pressure (1-5 bar) and low atomizer speed (6000-14000). Thus, abrasive powder like alumina when used in this new kind of spray dryer (in slurry form), erosion of nozzle and wheel is significantly reduced due to low slurry pressure and low wheel speed. Another novelty of the invented process is that besides selecting a suitable slurry composition, the grain/particle size distribution in the alumina powder can be varied at choice by varying wheel diameter and wheel speed and a typical particle size distribution can be attained either by increased speed with reduced wheel diameter or vice versa.
The non-obviousness in the process is the small particle size of the alumina powder (d90=1.5-4.7 μm) which can smoothly pass through the orifice of jet nozzle (0.6 mm diameter) in slurry form. Reduced viscosity of the slurry attained by addition of low cost chemical additive (dispersant) such as ammonium poly acrylate causes the small solid particles to homogeneously disperse throughout the volume of liquid such that clogging or chocking of jet orifice does not take place. The effect of dispersant and particle size of alumina powder (feed) contribute together to bring down the viscosity of the slurry to the range 2-100 cP, to the streamline flow range value of the carrier liquid, i.e. Water.
20-60 wt % alumina suspended slurry maintained at a low viscosity (2-100 cP) is injected at low pressure on to an atomizer wheel surface in the form of three jets. The rotating atomizer wheel disintegrates the slurry jets in to a large number of droplets at the wheel surface and the droplets are dried by hot air circulation (415-420° C.) inside a drying chamber. The outlet temperature of drying air is maintained at 95-110° C. Process parameters have been worked out to produce powder with particle size 37.5-105 μm (>59%) with sphericity more than 0.9. The powder is porous and found to be quite flowable (ffc: 17-18.5). The process shows >80% yield (by mass) and the product is found to exhibit good flowing performance in plasma torch for spray coating on substrate. The free flowing alumina powder finds good application ceramic industry in compaction, extrusion, manufacture of high tech and high density ceramic products, high temperature industrial components and parts, sintered products, refractories and castables. Also it finds wide use in making electronic component such as IC substrate, making cutting tools and wheels for mechanical engineering applications, and for coating thermal and insulating barriers on metal substrates.